Electrolytic cells such as Zirconium oxide are known to produce a logarithmic output signal indicative of changes in oxygen concentration differential on opposite sides of the electrolytic material. Such electrolytic cells are commonly used in process controls to detect, monitor, and control oxygen concentrations. Their use in control instrumentation requires that they provide a linear output signal requiring the linearization of the normally-produced logarithmic output signal.
In the past, attempts to linearize the logarithmic output signal of the electrolytic cell involved calibration utilizing three points; one at each end of the oxygen concentration range of the electrolytic cell, and a third point in the middle of this range. This generated a best fit straight line which tended to be S-shaped through these three points. The circuitry used to accomplish this linearization usually required three resistance adjustments for the three calibration points. The three resistance adjustments were usually interacting and the calibration required three different test gases for the three calibration points. Thus, the known linearization circuits involved calibration through interpolation rather than extrapolation.
Also, when an atmospheric reference system is used on one side of the electrolytic cell, such as Zirconiumoxide, the logarithmic output of the electrolytic cell reverses polarity at 20.9% oxygen. This fact requires complicated electronics since electronics cannot be easily made to follow such a polarity shift. All these problems resulted in complicated electronics which required calibration with three test gases and produced a relatively inaccurate linearization.
Thus, it can be seen that what was needed was a simple linearizing circuit for the logarithmic output of an electrolytic oxygen detector which would follow the polarity change of atmosphere-referenced electrolytic cells and which could be easily calibated, using less than three test gases.